<!-- 🚨 STOP 🚨 STOP 🚨 STOP 🚨 Half of all issues filed here are duplicates, answered in the FAQ, or not appropriate for the bug tracker. Even if you think you've found a *bug*, please read the FAQ first, especially the Common "Bugs" That Aren't Bugs section! Please help us by doing the following steps before logging an issue: * Search: https://github.com/Microsoft/TypeScript/search?type=Issues * Read the FAQ: https://github.com/Microsoft/TypeScript/wiki/FAQ Please fill in the *entire* template below. --> <!-- Please try to reproduce the issue with the latest published version. It may have already been fixed. For npm: `typescript@next` This is also the 'Nightly' version in the playground: http://www.typescriptlang.org/play/?ts=Nightly --> **TypeScript Version:** 4.1.0-dev.20200807 <!-- Search terms you tried before logging this (so others can find this issue more easily) --> **Search Terms:** conditional type tuple **Code** ```ts type Tuple<T> = [T extends null ? T? : T]; ``` **Expected behavior:** Expected the definition to work as expected. **Actual behavior:** Got the following error: > JSDoc types can only be used inside documentation comments. **Playground Link:** [Playground Link](https://www.typescriptlang.org/play?ts=4.0.0-beta#code/C4TwDgpgBAKgrmANhAPDAfFAvFA2jKCAD2AgDsATAZyjLkUSgH5YWAuWAXQG4AofgPQCoASSgALAIYA3aMAD2UCovnBxEAE4B3AJZUIvUJCgB5MMDSYc+Jj0PhoAJQgBHS9jww7R6AFlNAOaoGB4ExKSUNHQMzKbm7hzObhh8QA) **Related Issues:** Did not find anything close.